Background Pain in critically ill patients in the intensive care unit ( ICU) is common. However, pain assessment in critically ill patients often is complicated because these patients are unable to communicate effectively. Therefore, we designed a study ( a) to determine the inter- rater reliability of the Numerical Rating Scale ( NRS) and the Behavioral Pain Scale ( BPS), ( b) to compare pain scores of different observers and the patient, and ( c) to compare NRS, BPS, and the Visual Analog Scale ( VAS) for measuring pain in patients in the ICU. Methods We performed a prospective observational study in 113 non- paralyzed critically ill patients. The attending nurses, two researchers, and the patient ( when possible) obtained 371 independent observation series of NRS, BPS, and VAS. Data analyses were performed on the sample size of patients ( n = 113). Results Inter- rater reliability of the NRS and BPS proved to be adequate ( kappa = 0.71 and 0.67, respectively). The level of agreement within one scale point between NRS rated by the patient and NRS scored by attending nurses was 73%. However, high patient scores ( NRS = 4) were underestimated by nurses ( patients 33% versus nurses 18%). In responsive patients, a high correlation between NRS and VAS was found ( r(s) = 0.84, P < 0.001). In ventilated patients, a moderate positive correlation was found between the NRS and the BPS ( r(s) = 0.55, P < 0.001). However, whereas 6% of the observations were NRS of greater than or equal to 4, BPS scores were all very low ( median 3.0, range 3.0 to 5.0). Conclusion The different scales show a high reliability, but observer- based evaluation often underestimates the pain, particularly in the case of high NRS values (= 4) rated by the patient. Therefore, whenever this is possible, ICU patients should rate their pain. In unresponsive patients, primarily the attending nurse involved in daily care should score the patient's pain. In ventilated patients, the BPS should be used only in conjunction with the NRS nurse to measure pain levels in the absence of painful stimuli.